@ianmcd No there is no bubbling sound (kettling I believe it's called) when it does fire up and ignite. When it ignites it's a super boiler.
@topgazza We purchased the property a year ago and as such it was not us that had the boiler installed so the sale & install was not us. We paid for a one off repair as we don't yet have it on a service agreement and obviously the manufacturer won't touch us now until it's been power flushed. Mind you if their answer to ignition problems is to get the system power flushed I don't think I want them near my boiler again once they've fixed it.
The engineer that came yesterday took a water sample from the boiler (when I was out of the room) and showed me lots of sediment in the sample but interesting I have taken numerous (yes many) samples of water from the boiler drain point today and all have been spotless, i.e. no sign of any particles big enough to see with the naked eye. The samples I have taken have been after varying periods of non use (turned off and then after showers etc. I think his sample bottle must have been contaminated somehow, I wonder how that happened?